how to use XmlRpc::XmlRpcValue receive data in order

Question

i'm using XmlRpc::XmlRpcValue to receive data from .yaml file, but it seems that the received data order is different from the order in .yaml file.

here is my .yaml file:

wheelsets:
  left_front:
    position: [ 0.585, 0.557 ]  
    <<: &default
      steer_offset: 0.0
      wheel_radius: 0.1175
  right_front:
    position: [ 0.585, -0.557 ]
    <<: *default
  left_back:
    position: [ -0.585, 0.557 ]
    <<: *default
  right_back:
    position: [ -0.585, -0.557 ]
    <<: *default

and here is my code:

XmlRpc::XmlRpcValue wheelsets;
nh_.getParam("wheelsets", wheelsets);
ROS_ASSERT(wheelsets.getType() == XmlRpc::XmlRpcValue::TypeStruct);
for (const auto& wheelset : wheelsets)
{
    ROS_ASSERT(wheelset.second.hasMember("position"));
    ROS_ASSERT(wheelset.second["position"].getType() == XmlRpc::XmlRpcValue::TypeArray);
    ROS_ASSERT(wheelset.second["position"].size() == 2);
    ROS_ASSERT(wheelset.second.hasMember("steer_offset"));
    ROS_ASSERT(wheelset.second["steer_offset"].getType() == XmlRpc::XmlRpcValue::TypeDouble);
    ROS_ASSERT(wheelset.second.hasMember("wheel_radius"));
    ROS_ASSERT(wheelset.second["wheel_radius"].getType() == XmlRpc::XmlRpcValue::TypeDouble);
}

the code can run successfully, but the data i receive is in order of left_back, left_front, right_back, right_front, which i guess is alphabetical order.

Is there any way to receive data in order same as .yaml file?

Answer 1

With a hash, there should be no guarantee that the data will be in order.

If order is important, one would normally use a list.

Other options would be

std::vecgtor<std::string>sets = ["left_front", "right_front", "left_back", "right_back"];
for (const auto& s : sets)
{
    ROS_ASSERT(wheelset[s].hasMember("position"));
    ROS_ASSERT(wheelset[s]["position"].getType() == XmlRpc::XmlRpcValue::TypeArray);
    ROS_ASSERT(wheelset[s]["position"].size() == 2);
    ROS_ASSERT(wheelset[s].hasMember("steer_offset"));
    ROS_ASSERT(wheelset[s]["steer_offset"].getType() == XmlRpc::XmlRpcValue::TypeDouble);
    ROS_ASSERT(wheelset[s].hasMember("wheel_radius"));
    ROS_ASSERT(wheelset[s]["wheel_radius"].getType() == XmlRpc::XmlRpcValue::TypeDouble);
}